The present invention relates generally to gas turbine engines, and, more specifically, to boltless blade retainers in turbine rotor assemblies.
In a dual rotor gas turbine engine, a low pressure compressor or fan is driven by a low pressure turbine (LPT), and a high pressure compressor is driven by a high pressure turbine (HPT). Air is compressed in turn through the compressors and mixed with fuel and ignited in a combustor for generating combustion gases which flow downstream through the turbines which extract energy therefrom. Each turbine includes one or more stages of turbine nozzles and rotor blades. Each turbine nozzle includes a plurality of circumferentially spaced apart stator vanes which direct the combustion gases across respective rows of turbine blades. The turbine blades are mounted to the rim of a rotor disk using conventional circumferential entry or axial entry dovetails in corresponding slots in the rims of the rotor disks.
In the axial entry blade mounting design, the blade dovetails are inserted axially during assembly into corresponding axially extending dovetail slots in the disk rims. In order to axially retain the blades therein during operation, conventional blade retainers in the form of annular plates are provided at both the forward and aft ends of the disk in abutting contact therewith for axially trapping the blades. In early designs, the blade retainers were bolted to the rotor disks and therefore created undesirable effects such as stress concentration at the bolt holes. In typical modern designs, boltless blade retainers are used wherein a separate locking ring may be used to trap the blade retainers against axial movement thereof. Boltless blade retainers provide improvements in reduction of weight and windage losses, and most significantly eliminate the stress concentrations of the older design bolt holes.
Since turbine blades are typically cooled with a portion of compressed air bled from the engine compressor, the forward or upstream blade retainer typically includes a plurality of circumferentially spaced apart cooling air feed holes therethrough which channel the cooling air into the rim of the disk from which it is further channeled through the individual blades for cooling the airfoils thereof in a conventional manner. In this arrangement, the blade retainer is also referred to as a cooling plate and typically includes an annular seal wire disposed between the retainer and the disk rim for sealing the interface therebetween for channeling the cooling air through the disk rim without undesirable flow leakage.
Since the blade retainer is a discrete component which rotates with the rotor disk at relatively high rotational speeds during operation, it is suitably balanced for reducing or eliminating undesirable vibration therefrom. In a typical design, the blade retainer includes a radially inner hub having a radially outer diameter rabbet which engages a corresponding inner surface at a corner of the disk rim for preventing radial outward movement of the blade retainer. The outer rabbet is typically provided with a suitable interference fit with the rim corner for maintaining contact therewith under various operating conditions of rotational speed and thermal expansion and contraction.
More specifically, the rotor disk has relatively high mass compared to the blade retainer, with the blade retainer therefore thermally responding more quickly than the rotor disk due to the varying temperatures encountered during operation. In a speed burst for example, the rotor disk is accelerated up in speed, with the blade retainer being heated faster than the rotor disk. Accordingly centrifugal force and differential thermal expansion between the two components ensures that the outer rabbet is tightly maintained in contact with the rim corner for maintaining the originally provided balance.
However, during a speed chop in which the rotor disk is decelerated to a lower speed, the corresponding centrifugal force is reduced and, the blade retainer cools faster than the rotor disk which causes the blade retainer to contract at a greater rate than that of the disk. If insufficient initial interference fit and centrifugal force occur, the differential thermal contraction between the outer rabbet and the corresponding rim corner will cause radial separation or liftoff therebetween. In such an event the blade retainer would be allowed to move eccentrically causing rotor imbalance and undesirable vibrations. Furthermore, axial seal wire liftoff can also accompany radial rabbet liftoff causing undesirable cooling air leakage.
Since HPTs operate at greater speeds than LPTs, boltless blade retainers are typically found in the former but not in the latter. Rabbet liftoff can be avoided in designs that (1) have sufficiently high operational speeds with correspondingly high centrifugal loads; (2) use a blade retainer operated at conditions where the disk constrains and limits the retainer's radial deflection below that which it would achieve if operated off the disk; and (3) avoid use of the cooling air feed holes (when holes are implemented) which tend to separate the thermal response times of the blade retainer and the rotor disk. The high speed operating characteristics of the HPT provide a suitable environment for the implementation of boltless blade retaining cooling plates in which rabbet liftoff as well as seal wire liftoff are prevented as is conventionally known. As indicated above, rabbet liftoff leads to eccentric imbalance operation, with seal wire liftoff providing undesirable leakage of the cooling air from its intended flowpath.
Since LPTs operate at substantially lower speeds than HPTs, boltless blade retainers are typically not found therein. The use of a conventional boltless blade retainer in a typical LPT can result in the low operating speeds of the LPT locating the blade retainer in operating conditions which limit the ability to maintain contact between the outer rabbet and the rim corner. Since the blade retainer is generally capable of supporting itself at these conditions, the desired rabbet contact between the disk and the blade retainer is difficult to maintain. The relatively low centrifugal loads associated with a typical LPT idle speed are usually not enough to overcome the thermal contraction difference during the transient speed chop thus leading to separation of the outer rabbet and the disk rim causing imbalance, as well as undesirable seal wire liftoff. Rabbet liftoff is worsened when the blade retainer includes the cooling air feed holes. Accordingly, LPT blade retainer designs typically use bolted configurations in view of these operational considerations.